In plants, e.g. metallurgical plants, in which industrial gases (for example converter exhaust gases) must be scrubbed with water before these gases are released into the atmosphere, it has been found that a high efficiency in particulate and soluble-component removal can be gained by utilizing a so-called annular-gap washer.
Reference may be made to U.S. Pat. Nos. 4,218,241, 4,152,123, 4,055,331, 4,052,042, 4,007,025, 3,854,908, 3,976,454, 3,844,744, 3,844,745 and 3,726,065 which show gas-cleaning installations for various purposes which may utilize annular-gap washers or scrubbers of the type with which the present invention is concerned and/or which may deal with structural details of such scrubbers.
In general, the scrubber comprises a central body, generally in the form of a body of revolution, surrounded by a housing or casing defining an all-around clearance with a central body which may be axially movable to adjust the gap width and hence the pressure drop across the flow path formed between the body and the surrounding housing through the annular gap therebetween. A spray nozzle can discharge a water spray, preferably in atomized form, at a location axially spaced from the central body and usually upstream of the gap in the direction of gas flow. Generally the central body and housing are of circular cross section and the gap is likewise of circular cross section. Wash water or other scrubbing liquid is generally collected downstream of the gap and is found to contain a large proportion, if not all, of the components of the gas stream which are soluble in the scrubbing liquid and the particulates which are washed out of the gas stream by the liquid.
After separating contaminants, scrubbing liquid is generally recirculated to the spray nozzle or nozzles.
The mixture of dust-containing gas and water droplets or the aerosol of water is accelerated in the gap and brought into intimate contact, thereby permitting the scrubbing liquid to pick up the dust particles.
In general, the housing through which the mixture passes converges toward the body which can have a drop configuration or the configuration of a frustocone and, like the housing, can have a converging-diverging configuration. The housing can, downstream of the constricted portion, be divergent as well.
The device thus may be in the form of a so-called diffuser washer.
In a diffuser washer, the flow passage is in the form of a venturi nozzle with a progressively constricting inlet portion and an outwardly divergent discharge portion and the shape may be that of a Laval nozzle.
In most constructions of scrubbers of this type, the central body is disposed primarily in the region of the discharge portion and can be generally conical in this region. The small end of the generally conical body may project beyond the constriction into the inlet side of the housing.
Thus, while the gas stream is accelerated by the constriction upon entering the gap, within the gap itself the mixture is decelerated or suffers no change in velocity. The device can be so shaped and dimensioned that a substantial linear pressure drop will appear across the length of the flow passage, i.e. a constant pressure gradient is formed along the axial length of the flow passage.
The scrubbing effectiveness, i.e. the degree to which particles are removed from the gas and the particle size ranges of the removed particles, and the efficiency are functions of the pressure differential across the gap which can be varied by axially shifting the central body. This axial shifting of the central body allows, as described in some of the patents mentioned above, control of various process parameters.
The particle-removal effect is a function in part of mechanical forces and actions which are applied to the dust particles and droplets of the scrubbing liquid and which are superimposed upon the thermodynamic phenomenon. The gas may become saturated with water, e.g. the scrubbing liquid, and evaporation and condensation phenomena may occur, the condensation resulting in precipitation of water and waste particles to facilitate the removal thereof from the gas.
Conventional annular-gap scrubbers operate by causing liquid droplets to adhere to dust particles or dust particles to adhere to water droplets so that the resulting droplet is relatively massive and separates because of its kinetic energy, momentum or inertial action from the gas stream, e.g. upon subjecting the gas to a direction change downstream of the annular-gap scrubber.
Thus it can be seen that an annular-gap scrubber is basically an inertial separator whose function is determined exclusively by mechanical/thermodynamic properties.